PROJECT SUMMARY Nitrogen is an essential building block of life on earth; it is necessary for the structure and function of biomolecules such as proteins and nucleic acids, as well as commodity chemicals and fertilizers. The usable form of nitrogen needed for these processes is NH3, which is obtained by the six-electron reduction of N2. Nitrogenase enzymes are able to catalyze this transformation at room temperature and atmospheric pressure using the iron molybdenum cofactor (FeMoco) in MoFe nitrogenase. Though the amino acid surroundings of the cofactor play an essential role in enabling N2 reduction, little is understood about the specifics of how protons and electrons are transferred to N2. This is in large part due to the inability to poise the wild-type enzyme in intermediate reduction states. This prohibits our understanding of structural changes that may occur throughout the catalytic cycle, and limits spectroscopists and crystallographers who have attempted to isolate catalytic intermediates. Our work in this proposal seeks to establish the fundamental determinants of N2 activation and reduction in a small molecule model system that mimics several key features of the FeMoco. First, we incorporate a pendant hydrogen bond donor (HBD), whose location and identity can be varied to learn what orientations and distances are best to interact with iron-bound N2 or N2-derived intermediates. Second, our approach employs sulfur and carbon donors to the iron center, which provides a biologically-relevant coordination environment. We will use this innovative system to determine what electronic and geometric features are most effective for activating N2 towards reduction. The influences of the HBD will be quantified in a structure-activity relationship that correlates each component of the system to multiple dependent measures of N2 activation. We will perform similar analyses for ?NxHy? ligands that approximate intermediate stages during N2 reduction. Furthermore, our approach enables us to control the delivery of protons and electrons, thereby studying each step in significantly more detail than would be possible in the enzyme itself. Stoichiometric reduction studies will allow us to determine which steps during the conversion of N2 to NH3 are facilitated or inhibited by these second-sphere interactions. Ultimately, these studies will provide a systematic and rigorous means to establish the modes of activation that are feasible and realistic for N2 reduction by nitrogenase.